The invention relates to an exhaust gas probe for measuring properties of the exhaust gas of internal combustion engines and to a method of operating such an exhaust gas probe.
Such exhaust gas probes are for example concentration probes, for example oxygen concentration sensors, also known as lambda probes, as described for example in the publication Bosch Automotive Handbook, 25th edition 2003, pages 133 ff. Such probes may additionally be temperature sensors suitable for exhaust gas, gas-selective exhaust gas probes, which measure ammonia or nitrogen oxide contents for example, hydrocarbon sensors and particle mass or particle count sensors, for example soot sensors.
These probes have to be calibrated, i.e. design-related tolerances have to be compensated. This may be achieved by trimming the sensor element, for example by adjusting heating resistance, i.e. a desired function value, to a target value.
Once such probes have been adjusted accordingly, they are connected by means of a plug to an engine control unit, which reads out and processes the data detected by the probe.
Exhaust gas probes additionally exist in which changes to functions, for example characteristic drift, heating resistance drift or the like, over the probe's life have to be accepted, since there is no possibility of compensation. Finally, broadband probes are known (see abovementioned Handbook, pages 133, 134), in which pre-evaluation of the probe signal takes place in an adapter element.
It is also known to provide adjusting elements actually in the probe plug, for example a trimming resistor which is adjusted by means of a laser beam and which is connected in such a way that it forms part of a current divider in an evaluation circuit in the control unit. It is moreover also possible to carry out laser adjustment in the sensor element itself if the diffusion barrier is exposed or to grind off the diffusion barrier appropriately at its outer edges and the like.
None of the above-described measures straightforwardly enables optimum adaptation of the sensor properties or indeed of the ageing-related change to the sensor properties. All that is possible is to detect manufacturing-related tolerances on production of the sensor elements, for example heater cold resistance and function values. In some cases, adjustment is only possible using complex laser apparatuses and trimming processes. Adaptation of the sensor properties during operation of the sensor elements is not possible, however.
In particular, it is not possible to take account of any ageing or poisoning effects, which may for example arise with lambda probes. The same is true of ageing-related changes to the probe properties.